1. Field of the Invention
The present invention relates to a process for preparing a thin film of superconductor. More particularly, it relates to a process for depositing on a substrate a superconducting thin film of a compound oxide which is uniform in composition and has a higher transition temperature of superconductivity.
2. Description of the Related Art
Superconductivity is a phenomenon which is understood as a phenomenon of phase change under which the electrical resistance become zero and perfect diamagnetism is observed. Thus, under the superconducting condition, electric current of a very high current density can be delivered without any loss of power.
Therefore, if a superconducting power cable is realized, the power loss of the order of 7% which is inevitable in conventional power cables can be reduced greatly. Realization of superconducting coils for generating a very high magnetic field is expected to accelerate development in the field of fusion power generation in which electric power is consumed beyond the amounts used under the present technology, as well as in the field of MHD power generation or motor-generators. The development of superconductivity is demanded also in other industrial fields such as in the field or electric power generation; in the field of transportation, for example magnetic levitation trains, or magnetically propelled ships; in the medical field such as in high-energy beam radiation units; or in the field of science such as NMR or high-energy physics.
In addition to the abovementioned electric power applications, superconducting materials can be used in the field of electronics, for example, as a device using the Josephson effect in which quantum efficiency is observed macroscopically when an electric current is passed through a weak junction arranged between two superconducting bodies. Tunnel junction type Josephson devices, which are a typical application of the Josephson effect, are expected to be a high-speed and low-power consuming switching devices owing to the smaller energy gap of the superconducting material. It is also expected that the Josephson device will be utilized as high sensitivity sensors or detectors for sensing very weak magnetic fields, microwaves, radiant rays, or the like since variation of electromagnetic waves or magnetic fields is reflected in variation of the Josephson effect and can be precisely observed as a quantum phenomenon. Development of the superconducting devices is also demanded in the field of high-speed computers in which the power consumption per unit area is reaching the upper limit of the cooling capacity with increment of the integration density in order to reduce energy consumption.
However, their actual usage has been restricted because the phenomenon of superconductivity can be observed only at very low cryogenic temperatures. Among known superconducting materials, a group of materials having so-called A-15 structure show rather higher Tc (critical temperature of superconductivity) than others, but even the top record of Tc in the case of Nb.sub.3 Ge which showed the highest Tc could not exceed 23.2 K at most. This means that liquefied helium (boiling point of 4.2 K) is the only cryogen which can realize such very low temperature of Tc. However, helium is not only a limited costly resource but also require a large-scaled system for liquefaction. Therefore, there had been a strong desire for other superconducting materials having higher Tc. But no material which exceeded the abovementioned Tc had been found in all studies for the past ten years.
The possibility of the existence of a new type of superconducting materials having much higher Tc was revealed by Bednorz and Muller who discovered a new oxide type superconductor in 1986 [Z. Phys. B64 (1986) 189]
This new oxide type superconducting material is [La, Ba].sub.2 CuO.sub.4 or [La, Sr].sub.2 CuO.sub.4 which is called the K.sub.2 NiF.sub.4 -type oxide. The K.sub.2 NiF.sub.4 -type oxides show higher Tc as 30 to 50K which are extremely higher than the known superconducting materials and hence it becomes possible to use liquified hydrogen (b.p. =20.4K) or liquified neon (b.p. =27.3K) as a cryogen to bring them to exhibit the superconductivity.
However, the above mentioned new type superconducting materials which were just discovered have been studied and developed only in the form of sintered bodies or as a bulk produced from powders. The superconducting sintered bodies having a a bulk form inevitably contain non-reacted particles and hence are not uniform in composition and in structure, so that they can not be used for manufacturing electronics devices.
When the superconducting material is applied to a variety of electronics devices, it is indispensable to prepare a thin film of superconducting material. The superconducting thin film, however, can not be obtained if composition and structure of the thin film is not controlled precisely.
It is also expected to utilize the superconducting material to produce a superconducting elongated article comprising a supporting member made of metal or the like in a form of wire, strand, band, tape or the like and a superconducting thin film vacuum-deposited on the supporting member. In this case also, it is necessary to establish technology how to deposit the superconducting thin film in vacuum.
The vapor deposition technique has been used for producing a thin film of superconducting material such as Nb.sub.3 Ge and BaPb.sub.1-x Bi.sub.x O.sub.3. In case of a thin film of Nb.sub.3 Ge, particles of Nb and Ge are sputtered from several targets consisting of Nb and Ge, respectively, and are deposited onto a substrate to form a film composed of Nb.sub.3 Ge. Japanese patent laid-open No. 56-109,824 discloses a process for producing a thin film of BaPb.sub.1-x Bi.sub.x O.sub.3. by means of sputtering technique. But, no prior art has disclosed detailed conditions of physical deposition of the new type compound oxides. The present invention was completed after a variety of experiments and examinations which were done in order to overcome the abovementioned problem.
Therefore, an object of the present invention is to provide a process for producing a superconducting thin film which is uniform in composition and structure and has the higher critical temperature.